Hi there,
There are 3 rental agencies at the train station in Santiago, Avis, Hertz and National rental car. The last one, just before you go down the stairs. The station is about 15 minutes walk from old town.
There is a 3rd alternative,
http://www.easycar.es . They do not have an office, it is all online, but you pick up your car at the "National" rental agency as mentioned above. I think they have an agreement of sorts. I think all of these offer daily rates... I think... :?
... I think Sil mentioned another rental car site/place once that she had used, but I am not familiar with them.
As for how to drive out, I would recommend your to drive west, out to Noia then north along the coast. This road is really beautiful, you will pass by many fishing villages and beaches on your way. The road is "turny" as you go in-and-out of fjords along the coast. You will pass, Muros, one of my favorite places. Muros is a good place for lunch on the way out.
For the return, there is a more straight and faster (and more dull) way that goes from the coast, via Negreira, back to Santiago. You will see it when you study the map. To save time, take this route. To enjoy, take the route via Noia and Muros.
Enjoy!
Ivar